More than just referring about their behavior as individuals in the world, Paul is actually pointing to the way that they live out their heavenly citizenship with one another, their collective polity. See, Paul recognizes that as a local church, they have a common life with each other. That is, set in the context of their citizenship in the Roman empire. So Paul is calling out the Philippians to live out their common life together in the secular context, not by simply adopting the customs and the culture of the world around them, but rather to live out their collective life together in a manner that fits with the Good News of life in the reality of the Kingdom of God.

And the reason I think that's what he's referring to, is that the litmus test is unity. Paul will know that the Philippians are conducting themselves in a manner worthy of the gospel, because they are in unity with one another.

According to Paul, the main outcome of a local church conducting themselves in a manner worthy of the gospel—is that they end up standing firm, in one spirit, striving together, as one, for the faith of the gospel.